The functional significance of residual yolk in lizards

Residual yolk is assumed to be an important source of energy and nutrients during early life in nonmammalian amniotes.Available data show that the mean size of residual yolk is far smaller in lizards than in turtles,snakes,crocodiles,and birds,raising a question of whether residual yolk is of functional significance in lizards.Here,we compared data from 26 lizard species with those from other nonmammalian amniotes to test the hypothesis that residual yolk is functionally less significant in species producing more fully developed offspring.In our sample,species mean offspring water contents ranged from 73%to 84%of body wet mass;species mean proportions of carcass dry mass,fat-body dry mass,and residual yolk dry mass to offspring dry mass ranged from 84%to 99%,0%to 5.0%,and 0%to 14.4%,respectively.Lizards are,on average,more fully developed at hatching or birth than snakes,as revealed by the fact that the mean proportion of carcass dry mass to body dry mass and offspring water contents were both higher in lizards than in snakes.We conclude that the functional significance of residual yolk during early life is generally less evident in lizards.Even in the lizards where residual yolk is of potential functional significance,this portion of yolk contributes little,if any,to postembryonic growth.Future work could usefully collect data across a wider spectrum of reptile taxa to establish a precocial-altricial continuum and test the hypothesis that species with a smaller amount of residual yolk are closer to the precocial end of the continuum.

The geographical diversification in varanid lizards:the role of mainland versus island in driving species evolution

Monitor lizards(Varanidae)inhabit both the mainland and islands of all geological types and have diversified into an exceptionally wide range of body sizes,thus providing an ideal model for examining the role of mainland versus island in driving species evolution.Here we use phylogenetic comparative methods to examine whether a link exists between body size driven diversification and body size-frequency distributions in varanid lizards and to test the hypothesis that island lizards differ from mainland species in evolutionary processes,body size,and life-history traits(offspring number and size).We predict that:1)since body size drives rapid diversification in groups,a link exists between body size-driven diversification and body size-frequency distributions;2)because of various environments on island,island species will have higher speciation,extinction,and dispersal rates,compared with mainland species;3)as a response to stronger intraspecific competition,island species will maximize individual ability associated with body size to outcompete closely-related species,and island species will produce smaller clutches of larger eggs to increase offspring quality.Our results confirm that the joint effect of differential macroevolutionary rates shapes the species richness pattern of varanid lizards.There is a link between body size-driven diversification and body sizefrequency distributions,and the speciation rate is maximized at medium body sizes.Island species will have higher speciation,equal extinction,and higher dispersal rates compared with mainland species.Smaller clutch size and larger hatchling in the island than in mainland species indicate that offspring quality is more valuable than offspring quantity for island varanids.

Can snakes use yolk reserves to maximize body size at hatching?

We experimentally miniaturized freshly laid eggs of the Chinese cobra Naja atra(Elapidae)by removing^10%and^20%of original yolk.We tested if yolk-reduced eggs would produce 1)normal-sized hatchlings with invariant yolk-free body mass(and thus invariant linear size)but dramatically reduced or even completely depleted residual yolk,2)smaller hatchlings with normal­sized residual yolk but reduced yolk-free body mass,or 3)smaller hatchlings of which both yolk-free body mass and residual yolk are proportionally reduced.Yolk quantity affected hatchling linear size(both snout-vent length and tail length)and body mass.However,changes in yolk quan­tity did not affect incubation length or any hatchling trait examined after accounting for egg mass at laying(for control and sham-manipulated eggs)or after yolk removal(for manipulated eggs).Specifically,yolk-reduced eggs produced hatchlings of which all major components(carcass,re­sidual yolk,and fat bodies)were scaled down proportionally.We show that snakes cannot use yolk reserves to maximize their body size at hatching.Furthermore,our data also suggest that the parti­tioning of yolk in embryonic snakes is species-specific.

Contributions of source population and incubation temperature to phenotypic variation of hatchling Chinese skinks

Phenotypic plasticity and local adaptation are viewed as the main factors that result in between-population variation in phenotypic traits,but contributions of these factors to phenotypic variation vary between traits and between species and have only been explored in a few species of reptiles.Here,we incubated eggs of the Chinese skink(Plestiodon chinensis)from 7 geographically separated populations in Southeast China at 3 constant temperatures(24,28,and 32℃)to evaluate the combined effects of dutch origin,source population,and incubation temperature on hatchling traits.The relative importance of these factors varied between traits.Nearly all examined hatchling traits,including body mass,snout-vent length(SVL),tail length,head size,limb length,tympanum diameter,and locomotor speed,varied among populations and were affected by incubation temperature.Measures for hatchling size(body mass and SVL)varied considerably among clutches.Source population explained much of the variation in hatchling body mass,whereas incubation temperature explained much of the variation in other examined traits.Our results indicate that between-population variation in hatchling traits of P.chinensis likely reflects the difference in natural incubation conditions and genetic divergence.